Transducers



Nov. 25, 1958 Filed Jan. 21, 1953 s. KELLY 2,862,068

TRANSDUCERS 2 Sheets-Sheet l 3o 73 l5 l9 2/ Inventor.

, AH-orn eys.

Nov. 25, 1958 s. KELLY 2,862,068

TRANSDUCERS Filed Jan. 21, 1953 2 Sheets-Sheet 2 Inventor:

1 KM, v 46 AH'OrD eys.

United States Patent G TRANSDUCERS Stanley Kelly, Enfield, England, assignor to Cosinocord Limited, Enfield, England, a British company Application January 21, 1953, Serial No. 332,177 Claims. (Cl. 179--100.41)

This invention relates to transducers of the type employing a piezo-electric transducing element having electrodes upon it; the element is restrained at at least one part of its surface, and an alternating mechanical force is applied to it at another, whereby a voltage is produced upon the electrodes. Very often the element is of a bimorph construction, in which two plates of the piezoelectric material, of different orientation as to their crystal axes, are secured together.

The nature of the restraint imposed upon the element has a very important effect upon the response characteristic of the transducer, considered as the voltage output for a given applied force over a range of frequencies. For transducers which are to be used in gramophone pickups and like devices it has been found that satisfactory results can be obtained by the use of a restraining material in the form of a viscous gel in which the element is immersed. Such a gel must be chosen to have a resistance and compliance such that when a force of relatively low frequency is applied to the element the gel oiffers a predominantly compliant restraint, whilst at higher frequency the frictional component of the material is such that the restraint is determined predominantly by the mass of the gel. The electrical analogue of the gel for such conditions is that of a resistance in series with a capacitance.

The gel suggested for this purpose has been of a semiliquid nature and its insertion in the casing of the transducer is a somewhat messy operation; moreover, the use of a semi-liquid material, though this hardens sufliciently in time to be non-fluid, increases the difliculty of assembly.

The present invention has for its object to provide an improved method of constructing an electrical transducer of this kind and in accordance with the invention there is formed a sheath of material of appropriate mechanical characteristics, into which the piezoelectric element is inserted so as to provide contact with the element over a substantial area.

The invention will be better understood by the fOlIOWe ing description of one embodiment thereof, as applied to a gramophone pickup intended for use with. so-called Standard 78 R. P. M. records or Long-playing 33 /2 R. P. M. records, in conjunction with the accompanying drawing in which:

Figure l is a side view of the pickup head embodying the invention, with the cartridge in section, in the safe postion.

Figure 2 is an underplan of the same pickup head, but with one side of the casing of the cartridge removed.

Figure 3 is a perspective view of the moulded constraining and damping pocket for the crystal.

H 2,862,068 Patented Nov. 25, 1958 crystal is located within a moulded constraining and damping pocket 13 which will be further rescribed hereinafter, of rubber or other material, whilst the forward edge of the crystal is gripped in a yoke 14 having a bearing pivot 15 mounted in a rubber or like bushing 16. Terminal lugs 17 are provided on the casing, with flexible leads 18 leading to the electrodes provided on the crystal in the usual way; when the forward end of the crystal is vibrated, i. e. oscillated about the axis of bearing pivot 15 there will thus be a corresponding voltage produced at terminals 17.

The yoke 14 has coupled to it two cantilever arms 19 and 20, carrying respectively styli 21 and 22. These cantilever arms are carried by arms 23 and 24, which are attached to yoke 14. As shown, the outer ends of the arms 19 and 20 are bored to receive the ends of the cantilever arms, which are then stuck or soldered in position; the inner ends of the arms have semicircular extensions 23a and 24a respectively, which pass into a circular boring in yoke 14 and are there held by screw 25.

The cartridge is mounted so that either stylus 21 or 22 can be brought to the underside of the pickup. With this object the two parts 10, 11 of the case are secured together by rivets 26 and 27; rivets 27 at the front of the casing also secure a mounting member 28. This member has a forwardly extending shaft which enters and projects through a bush 29 fixedin a bracket 30. A compression spring 31 surrounds the shaft, and is retained by a small knob 32. The abutting faces of the bush 29 and the pivot member are formed to provide a detent mechanism which locates the cartridge with one or other downmost, and also, in the manner described in a copending application No. 24,83 8/52, in a position in which the cartridge can be held horizontal when not required for use. It will be seen that the mounting member also has in it a recess to receive the rubber or like bushing 16.

The response which can be obtained with a piezoelectric crystal element when it is driven at varying frequency depends upon a variety of factors which, as is well known, includes the mechanical characteristics of the moving part of the system. In addition a very important factor is the nature of the constraint which opposes the motion of the crystal in response to the applied force.

In accordance with the present invention the constraint upon the crystal is obtained by means of the moulded sheath 13. This sheath, as appears from Figures 1, 2 and 3, has a generally rectangular shape with an internal rectangular opening 13a extending from one end to the other, so that the sheath has .the appearance of a fiat rectangular tube. The internal dimensions are such that the sheath willfit closely upon the crystal bimorph but the thickness of the sheath, that is to say the distance between its two major rectangular surfaces, is slightly less than the corresponding dimension of the case. On the edges of the sheath are provided small lugs or projections 13b which project both vertically and horizontally from the sheath at points spaced laterally from and on opposite sides of the axis of rotation of said crystal, with the result that when in position in the casing 10, 11 the major surfaces of the sheath are not in contact with the adjacent surfaces of the casing. At its one end the sheath is recessed at 13c to accommodate the claw 14.

It has already been mentioned that the nature of the constraint upon the crystal has an important bearing upon the response obtained; the effect may be better understood by reference to Figure 6 which is the electrical analogue, to a first order of approximation, of the piers-electric transducer described.

In Figure 6 the input, corresponding to the input from the record, is shown as a generator V, shunted by a capacitor C representing the compliance offered to the stylus by the record surface. The stylus has a mass, represented by inductor L and compliance represented by capacitor C The stylus not in use has mass and compliance represented by L and C and the yoke has mass represented by L The material of the sheath is chosen to have a frictional loss, represented by resistor R and a compliance, represented by capacitor C The moving parts including the sheath and the crystal in effect are a transmission line with distributed mass and compliance, but to a first order these can be represented by inductors L L and L and capacitor C That part of the sheath between the crystal and the casing can be represented by a capacitor C and resistor R this part having both frictional loss and compliance. Transformer T, represents the mechanical transformation ratio of the crystal, and C the electrical capacitance.

in considering the response of this impedance network to an applied voltage of increasing frequency it will be seen that L and C due to the stylus form a series resonant circuit; the resonant frequency of this circuit will depend not only upon the compliance C but also upon those elements which are effectively in shunt to it. As a result of this, in a practical case the resonant frequency was in the region of 1-2 kcs./sec. At this frequency the motional impedance of the stylus is a minimum, since looked at from the input terminals the network is series resonant. The impedance at resonance will be determined by the resistance, that is the friction, in the system, due mainly to R and R It is to be observed that there is also a secondary resonance due to C C L L and L damped by R This is the resonant frequency of the crystal and the sheath. This resonant circuit is dependent upon both the compliance C; of the coupling between the yoke and the crystal and also upon the compliance of the constraint imposed upon the crystal and represented by R and C In a practical case the resonant frequency of this part of the system, ignoring the compliance of the constraint, was about 5.2 kcs./sec. The unused stylus, represented by L and C behaves as a high impedance resonant circuit of a frequency in the region of 3.5 kcs./sec., the magnitude being determined by L and C and the frictional losses. This tends to produce a small dip in the characteristic, but is not diflicult to control.

The choice of termination represented by R and C is very important in controlling the nature of the response curve of the pickup. As pointed out in a copending application, the output which can be obtained from a piezoelectric crystal in response to a force applied to it depends very largely upon the manner in which the crystal is constrained to resist the applied force. For example, if a crystal is constrained only by inertia of its own or an applied mass, the output voltage will be proportional to the acceleration with which the crystal is driven. In the case where one end of a crystal is clamped in an infinitely rigidclamp, and the crystal is moved by a force applied to its other end, the voltage produced will be proportional to the amplitude of deflection caused by the force. This is also true in the case in which the end of the crystal is constrained by means presenting a pure mechanical compliance. If the crystal is constrained by a mechanical resistance, i. e. by a material exhibiting friction but no stiffness the output is proportional to the velocity with which the crystal is driven.

The standard recording frequency characteristic for disc records employs constant amplitude recording for frequencies up to a turnover point in the region of 250 C. P. S. and constant velocity recording above that frequency. if a pickup such as that described above is used with a recording of this characteristic the overall frequency response will clearly depend upon the nature of the constraint upon the crystal. Considering the types of constraint mentioned above, for example, with a mass controlled crystal the output will increase with frequency, with a slope of 12 dbs per octave, up to the region of the turnover frequency and above that frequency it will continue to increase with frequency, but at a slope of 6 dbs per octave. Where the crystal is rigidly clamped, or stiffness-controlled, the output characteristic is level up to the turnover point, and above that frequency falls with a slope of 6 dbs per octave. In the third case, with the resistance-controlled crystal, the output rises at 6 dbs per octave up to the turnover point, and above that frequency is uniform. These considerations apply only up to the resonant frequency of the moving system.

in the construction described it is arranged that the constraint upon the crystal is partly compliant and partly frictional, so that a level response is obtainable over a wide range of frequencies. it is, of course, true that all practical materials which are used to constrain a crystal element will be both frictional and compliant to some extent, though chosen to be compliant or frictional as the case may be. For the purpose of the present invention a suitable material can be defined by the dissipation constant (ZWICR) of the material, and should be approximately 6 for the present standard turnover frequency of 250 C. P. S.

Expressed in another way, this means that time constant of the material used for the clamping is equal to the period of one cycle at the turnover frequency. It is also arranged that the impedance of the compliance C is made equal to the resistance of R at a frequency some what below the turnover frequency. As a result of this choice of material the characteristic of the transducer element follows, up to the turnover point, the characteristic of the stiffness-controlled crystal and above the turnover point the characteristic of the resistance-controlled clamp. The overall characteristic is thus uniform with frequency.

All these various considerations affect the frequency response of the pickup as measured by the voltage output in response to mechanical vibration of the stylus, but as regards the design of the sheath 13, the following points will be seen:

Firstly the compliance of the sheath represented in Figure 6 by the capacitance C is concentrated very largely in the projections 13b, and as a result this compliance can be controlled very easily by variation of the dimensions of these projections, and without greatly altering the other mechanical impedances. Again, as pointed out in a copending application and as mentioned above, it is an advantage if the time constant of R C combination is made equal to the period of one cycle at the turnover frequency, that is, equal to about 4 milliseconds. The magnitude of C can be controlled as mentioned above; the magnitude of R can be controlled by the nature of the material which is used for the moulding of the sheath. Suitable materials are referred to hereinafter.

The resistance R which damps the main resonance of the moving system is also due to this same material, but the magnitude of R can be controlled by controlling the area of material of the sheath in contact with the crystal.

In the sheath 33 shown in Figure 4, for example, the same general shape as that of Figure 3 is adopted but the sheath is shorter. In sheath 33 the projections 33a, similar to projections 13b are used and in addition projections 33b are formed on the sheath. Projections 33b are located near the axis of rotation of the crystal and for this reason their contribution to the mounting compliance C is small, but they assist in mechanically supporting the crystal in the casing and thus reducing the possibility of damage.

Figure 5 shows a sheath 34 Which shows a method of controlling the magnitude of resistance R without substantially varying the resistance R This sheath is similar to sheath 33, and has projections 34a and 34b corresponding to projections 33a and 33b but in sheath 34 the corners of the sheath are cut away having a portion of the crystal (not shown in Figure 5) with damping material in contact with it. In the sheath of Figure 5 is shown also a method of increasing the compliance C without greatly affecting the remaining elements. In sheath 34 the central opening 34c to receive the crystal is enlarged at 34d at the sides near to projections 34a.

The material of the sheath must have appropriate frictional loss and compliance and must be capable of being moulded.

The constraining material chosen in this particular example consists of polyvinyl chloride in compound with an allyl alcohol ester of diabasic acids such as phthalic acid, succinic acid or maleic acid, and with or without the addition of other plasticisers such as for instance tricresyl phosphate dibutyl phthalate or dioctyl phthalate and with or without added fillers, pigments, lubricants, and heat stabilisers.

The allyl alcohol ester of dibasic acids such as for instance diallyl phthalate is preferably used as a partial polymer to achieve the required resistive properties.

For instance diallyl phthalate of a purified grade is partly polymerised with the aid of a catalyst such as benzoyl peroxide to give to of a prepolymer in solution of monomeric diallyl phthalate of sp. g.. 1.13 to 1.18.

The partial polymer of diallyl phthalate is compounded with the polyvinyl chloride resin and with the additives in the usual manner such as for instance by passing the mix over a heated compounding mill.

The following is an example of a suitable material. The proportions are parts by Weight:

Example 1 Polyvinyl chloride resin to 45 Diallyl phthalate prepolymer in solution of its monomer sp. g. 1.13-1.18 to 55 Tricresyl phosphate 15 to 20 Pigments 2 to 2.50 Basic lead carbonate 0.50 to 0.80

Example 2 Vinyl chloride-vinyl acetate copolymer resin containing 85% vinyl chloride, molecular weight 6,000 to 8,000 28 to 36 Polybutyl acrylate resin 60 to 75 Lead stearate 0.50 to 0.70 Pigments or fillers 1.50 to 2.50

In the particular pickup described the mechanical elements had the following values:

Mass (Kilogram-metre 10-1o The device in accordance with the invention and particularly the embodiment described, has the further ad vantage that in comparison with the known arrangement, the compliance of the mounting can be considerably increased for a given robustness of the system. This has a two-fold advantage; the pickup will track on a record with a lower vertical needle pressure, and secondly, there is less danger of fracturing the crystal due to excessive movement of the stylus.

I claim:

1. A gramophone pickup comprising a casing, a piezo electric element mounted in said casing so as to be capable of vibration with respect thereto about an axis extending through said element, means for vibrating said element and means for constraining said element about said axis to resist said vibration and thereby to produce a piezo-electric output, said constraining means consisting of a moulded sheath of material embracing said ele ment and the main body portion of said sheath being spaced from the inner wall of said casing, said sheath having spaced projections extending from the surfaces thereof to and engaging the inner Wall of said casing at points spaced laterally from and on opposite sides of said axis of vibration of said element.

2. A pickup in accordance with claim 1 wherein said projections are constituted by lugs projecting both vertically and horizontally from said sheath.

3. A pickup in accordance With claim 1 wherein said projections are constituted by lugs disposed at the sides of said sheath, said lugs extending longitudinally such that the ends thereof engage the inner faces of one pair of opposite walls of said casing and] the sides of said projections engage the inner faces of another pair of opposite walls of said casing.

4. A pickup according to claim 3 and wherein said sheath includes additional projections on opposite major faces thereof located near the axis of rotation of said piezo-electric element and which also engage respectively the inner faces of opposite walls of said casing.

5. A pickup according to claim 4 wherein said sheath includes an opening having a central portion engaging the major faces of the piezo-electric element, the side portions of said opening being enlarged.

References Cited in the file of this patent UNITED STATES PATENTS 2,463,109 Jafie Mar. 1, 1949 2,594,948 Lynch Apr. 29, 1952 2,668,196 Bauer Feb. 2, 1954 FOREIGN PATENTS 158,364 Australia Feb. 2, 1951 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,862,068 November 25, 1958 Stanley Kelly requiring correction ed below.

Column 6, lines 37 and 38, strike out "about said axis" and insert the same after "element" first occurrence, in line 37, same column,

Signed and sealed this 10th day of March 19590 (SEAL) Attest:

KARL H, AXLINE ROBERT C. WATSON Attesting Oflicer Commissioner of Patents 

